Why does the author mention that Renata's phone battery was at eighty percent at noon?

Renata Gonzalez had agreed to the bird count for one reason. Her mother had promised that if Renata drove her little brother to the wildlife refuge before sunrise on a Saturday in December, she could have the car for the rest of the weekend.

Mateo was eleven and had been ready since five. He sat in the passenger seat with a notebook on his knees, binoculars around his neck, and a field guide so worn that the cover was held on with tape. "The green jays will be near the feeders," he said as they pulled into the parking lot outside Alamo. "They're loud, so you'll hear them first."

Renata planned to sit on a bench and answer messages until it was over. The volunteer leader, a retired science teacher named Mrs. Salazar, had other plans. She handed Renata a clipboard and a pencil. "You're our recorder," she said. "Every bird anyone calls out, you write down. Species and number. If we can't agree, you write that down too."

For the first hour, Renata wrote whatever the others told her. Then a flash of color crossed the trail in front of her, green and yellow with a bright blue head, and before she could think about it, she said, "Green jay. Two." Mateo turned and stared at her as if she had suddenly started speaking another language.

"Three," Mrs. Salazar said, pointing. "Look at the branch behind them."

Renata looked. She changed the number.

By noon her phone battery was at eighty percent, the same as when they had arrived. The clipboard held four pages of careful pencil marks: kiskadees, two ospreys over the lake, a long line of white ibis.

On the drive home, Mateo fell asleep with the field guide open on his chest. At a red light, Renata reached over and turned the pages until she found the green jay. She read the description twice. Then she took a pen from the cup holder and, in the small white space beside the picture, wrote the date and the number three.